Adding Languages to Writer's Character, Font Menu

How do I add a language to this menu so that fonts that can will render
text in the style appropriate to the language?  I am reconciled to
having to create a bespoke version of LibreOffice, though I'd rather
not.

Manually editing a document's XML files would be the last
resort - it seems to work!  While it gets the language into the pick
list, this is only while the selection includes text in that language.
I haven't explored this method, though it does suggest a workable clumsy
technique.

An example is a Tai Lue style (language khb-CN).  LibreOffice (more
precisely, HarfBuzz) includes data to enable the conversion of 'khb' to
OpenType tag 'XBD '.  I'm styling Lanna script glyphs for language, and
selecting Lao as the complex-script language gives me Lao-style glyphs
from the font in LibreOffice.

The only mechanism I can see that might work for Tai Lue is to request
the installation of a scrappy dictionary for the language (perhaps even
empty?) but this method feels wrong.  My wish list for additions,
assuming I only have to include language and country, is:

khb-CN  Tai Lue
nod-TH  Northern Thai
kkh-MM  Tai Khuen
tts-TH  North-Eastern Thai

and, lower down the scale of desire, support for

khb-LA  Tai Lue of Laos

and for various Palaung languages (relevant one unestablished), all
OpenType code PLG:

pce-MM – Ruching
pll-MM – Shwe
rbb-MM – Rumai

If script matters, it gets more complicated.  The above 8 are all for
the Lanna script, but for generally useful dictionary support for
some of the languages one should concentrate on:

nod_Thai-TH
tts_Thai-TH
khb_Laoo-LA

and I'm not sure which script for Palaung - probably Myanmar, but
definitely not Lanna. 

A dictionary for pi_Lana (Pali) would be good to have - I'm not
sure about the the relevance of national variations, though.  I'm not
sure how well a multiscript dictionary will work.  A pi_Latn dictionary
is reported to have been developed, but it's not available for
download.  Pali is on the pick list for Western scripts, but is not
available for 'complex' scripts.  The national variations in Pali
generally go with script. (Of course, there are two very different, but
apparently equivalent, orthographies for pi_Thai-TH.)

Richard.